Generally, a compression type refrigerator system is composed of a compressor, a condenser, an expansion valve and an evaporator, having a mechanism whereby a mixture of a refrigerant and a lubricating oil is circulating in the closed system. In said compression type refrigerator system, the temperature in the compressor generally rises to 50.degree. C., while the temperature can also come to be of -40.degree. C. or so, though it depends on the kind of apparatus. Accordingly, the refrigerant and the lubricating oil must circulate in this system without phase separation in the usual range of -40 to +50.degree. C. If a phase separation occurs while the refrigerator system is running, it seriously affects the life and efficiency of the apparatus. For example, if phase separation of the refrigerant and the lubricating oil occurs in the compressor, the moving parts would be inadequately lubricated, resulting in seizure or other troubles, and thereby the life of apparatus would be shortened considerably. If phase separation occurs in the evaporator, the viscosity of the lubricating oil increases and thereby the efficiency of heat exchange is decreased.
Since a lubricating oil for refrigerator systems is used for the purpose of lubricating the moving parts of the refrigerator system, its lubricating properties are also important as a matter of course. Since the temperature becomes very high, particularly in the compressor, the oil is required to have a sufficient viscosity to retain the oil film necessary for lubricating.
The necessary viscosity varies with the kind of operating conditions of the compressor, but usually, the kinematic viscosity of the lubricating oil before mixing with a refrigerant is preferably 2 to 50 cSt at 100.degree. C. If the kinematic viscosity is lower than the above, the oil film becomes thinner and thereby seizure is liable to arise, while if it is higher, the efficiency of heat exchange is decreased.
Heretofore, chlorofluorocarbons (CFC) including dichlorodifluoromethane (hereinafter referred to as R-12) has often been used as the refrigerant for compression type refrigerator systems, and various mineral oils and synthetic oils have been used as the lubricating oil, satisfying the required properties described above. R-12, however, has recently been restricted more and more severely all over the world, for the concern of environmental pollution problems, that is depletion of the ozone layer. Therefore, hydrofluorocarbons including R-134a have come to be noticed as possible refrigerant. Said hydrofluorocarbons, particularly R-134a have little possibility of depleting the ozone layer and can substitute for R-12, with minimal changes in the structure of the conventional refrigerator systems. Accordingly, it is presently preferred as a refrigerant for compression type refrigerator systems.
When hydrofluorocarbons including the above R-134a are employed as the refrigerant for compression type refrigerator systems instead of R-12, the desirable lubricating oils come to be those having high miscibility with said hydrofluorocarbons including R-134a, and also having high lubricating properties to satisfy the required properties described above. However, since the conventional lubricating oils which have been used with R-12 do not have good miscibility with hydrofluorocarbons including R-134a, a new lubricating oil suitable for said compounds is required. Further, particularly in air conditioners for automobiles, it is required that the equipment should preferably not be modified on the substitution for R-12.
Therefore it is not desirable to have to modify the present equipment because of phase separation due to lack of compatibility of the usual lubricating oils with HFC. Accordingly, a lubricating oil having very favorable miscibility with hydrofluorocarbons including R-134a is required.
As lubricating oils having miscibility with R-134a, polyalkyleneglycol compounds are known in the art, such as e.g. Ulcon LB-165 and Ulcon LB-525 (Ulcon is a tradename of Union Carbide Co., Ltd.).
Also, oil compositions for refrigerator systems with a high viscosity employing polyoxypropyleneglycol monobutyl ether as a base oil have been known (Japanese Patent Publication No. 42119/1982).
However it is known that such lubricating oils, which are polyalkyleneglycol derivatives having polypropyleneglycol with hydroxyl group at one terminal and an n-butyl ether bond at the other terminal, do not have sufficient miscibility with R-134a, and for example, Ulcon LB-525 described above is known to cause phase separation with R-134a at room temperature (U.S. Pat. No. 4,755,316).
On the other hand, polyoxyalkylene glycol having at least two hydroxyl groups in a molecule is proposed to be a favorable substance miscible with R-134a (U.S. Pat. No. 4,755,316). However the resulting refrigerant-lubricating oil compositions based on these compounds do not have the required properties.
U.S. Pat. No. 4,428,854 claims an absorption refrigerant composition comprising R-134a and at least one organic solvent selected from the group consisting of tetraethylene glycol dimethyl ether, dimethylformamide, methyl ethyl ketone and (methyl) or (ethyl) or (butyl) tetrahydrofurfuryl ether. Said absorption type refrigerator systems, however, are quite different in mechanism from the compression type refrigerator systems described above, and tetraethylene glycol dimethyl ether described in the Examples of the above patent is not proper as a lubricating oil for compression type refrigerator systems if just because of its particularly low viscosity.
Thus, lubricating oils for compression type refrigerator systems having sufficiently good miscibility with R-134a and high lubricating properties have not been found yet, and their development has been eagerly desired.
An essential requirement of the lubricating oil for refrigerator systems is a wide temperature range for compatibility with the refrigerant (compatibility) which means no clouding at high temperature (high salvation of the refrigerant to the lubricating oil), high molecular polarity of the lubricating oil and no separation of the lubricating oil from the refrigerant at low temperature (high solubility of the lubricating oil to the refrigerant, and low molecular weight of lubricating base oil). For this reason, high compatibility with the refrigerant is very important, and it is necessary that the substance is not separated from the refrigerant at high or low temperatures and that it does not react with it. If the compatibility with the refrigerant is low, the equipment may seize on the portion of the refrigerator system which is subject to high temperature.
The man in the art knows how to determine and how to obtain the suitable viscosity of a lubricating oil in function of the type of refrigerator system used.
Further, the lubricating oil for a refrigerator system should not be corrosive to the refrigerating equipment, should preferably not reduce its insulating properties, and has to have high stability to the refrigerant.
Also, the lubricating oil for a refrigerator system should improve the wear-resistant properties of aluminum components in piston and bearings, made of iron and aluminum, of the refrigerator system.
An object of the present invention is to provide a lubricating oil for a refrigerator system suitable for the use in the refrigerator system using refrigerants such as hydrofluorocarbons including 1,1,1,2-tetrafluoroethane, having a wide compatibility temperature range with the refrigerant.
Another object of the present invention is to provide a lubricating oil having no corrosive properties to refrigerating equipment.
Yet another object of the present invention is to provide a lubricating oil showing little or no reduction in insulating properties.
A further object of the present invention is to provide a lubricating oil having a high stability to the refrigerant.
Yet a further object of the present invention is to provide a lubricating oil improving the wear-resistant properties of equipment components.
Still a further object of the present invention is to provide a lubricating oil suitable for use in compression type refrigerator systems using hydrofluorocarbons and particularly R134 or R134a.